A helicopter had just taken off when there was a loud bang and the engine started to overspeed. After landing and inspection, the transmission was disassembled. It was discovered that the assembly containing the output shaft to the main rotor had failed. The output shaft assembly was made up of two parts: the output shaft with an integral 10 in. diam upper disc at approximately mid-section; and a 10 in. diam lower disc. During manufacture, the lower disc was attached to the output shaft by an electron beam weld. The fracture had a single fatigue initiation site, coincident with...